Pure polystyrene such as General Purpose Polystyrene (GPPS), Expandable Polystyrene (EPS) or Oriented Polystyrene (OPS), a vinyl aromatic polymer, is considered a brittle polymer. The preparation of modified vinyl aromatic polymers such as modified polystyrene to alter its physical and mechanical properties is known. An example of conventional rubber-modified polystyrene manufacture is disclosed in U.S. Pat. No. 4,271,060. Typical modification processes can include a bulk, suspension or continuous process in which one or more additional monomers or polymers are combined with the vinyl aromatic. With polystyrene, current commercial products include a modified polystyrene polymer based upon either grafted conjugated diene rubbers (high impact polystyrene, HIPS); or physical blends of anionically-produced thermoplastics, such as, styrene-butadiene-styrene (SBS) block copolymers and general purpose polystyrene (GPPS) that yield transparent impact polystyrene (TIPS). Current HIPS products are not transparent.
Various thermosetting polyester compositions are known which, when molded, exhibit desirable mechanical properties. These composites will be referred to as unsaturated polyester resins (UPR). Polyester resins are widely used in molding applications in liquid form. Such liquid resins comprise a liquid solution of a liquid or solid polyester dissolved in a liquid crosslinking agent such as, for example, styrene. Commercial polyester resins usually contain 35-45% styrene, by weight. All percentages given herein are weight percent unless otherwise specified. Optimum physical properties are obtained around this level. The liquid polyester resin can optionally further include additional components including but not limited to glass fiber reinforcements and/or fillers, such as calcium carbonate or talc, free radical sources, such as peroxides, low profile additives (LPA), pigments, thickeners, inhibitors, toughening agents, release agents, and other components, as will be evident to those skilled in the art. Still, these UPR composites are typically brittle. This brittleness can often lead to catastrophic part failure. Furthermore the brittle parts can develop micro-cracks that detract from the quality of the surface and lead to problems with painted UPR composite parts. The low profile additives are used, in part, to prevent shrinkage. However, shrinkage of UPR composite parts still presents difficulties for fabricators.
The UPR curing reaction is usually initiated by the addition of a peroxide catalyst. Polar monomers and additives are sometimes used in these polyester compounds to improve paintability.
The mixing of different polymer types can dramatically change the physical properties of the resulting materials. For example, the addition of a “rubbery” material such as butadiene to “brittle” polystyrene changes its impact resistance. It is generally accepted that the morphology of these blends dictates the final bulk properties. Furthermore, it is known that most polymer pairs are immiscible due to the minimal entropy of mixing associated with large molecules. Thus blending polymers having different structures prevents thorough mixing and phase separation occurs. This phase separation can lead to poor properties and lack of morphology control. Block copolymers physically mixed into other polymers of similar structure are known to be compatible mixtures. Block copolymers are most commonly produced via living anionic polymerization techniques. Polymers produced via such techniques typically have chain ends that are terminated with non-reactive groups and cannot be used to subsequently reinitiate further polymerization reactions.
To improve the impact resistance of polystyrene, 5-10% of a conjugated diene rubber (e.g. polybutadiene) can be added to the polymerization mixture. In the course of the polymerization, the conjugated diene polymer is grafted onto the polystyrene which results in higher impact properties. The general commercial name for such materials is high impact polystyrene (HIPS). High impact polystyrene is opaque.
Another method of producing impact modified polystyrene is to physically blend polystyrene and a styrene-butadiene-styrene (SBS) block copolymer elastomer. Styrene-butadiene-styrene (SBS) block copolymer is typically produced anionically then physically blended with styrene in an extruder in a range of from about 25% to 75% SBS. The general commercial name for the blend is transparent impact polystyrene (TIPS). One primary disadvantage for blending polymers to form TIPS is cost, as it requires the separate manufacture of polystyrene and a rubber material followed by granulation and processing. Acrylic polymers of low Tg (glass transition temperature), like polyethylhexyl and polybutyl acrylate, are rubbery in nature. However, physical blends of acrylic polymers such as polybutyl acrylate and polystyrene are not compatible.
U.S. Pat. No. 5,721,320 discloses a free radical bulk polymerization process for producing a rubber modified vinyl aromatic comprising polymerizing the vinyl aromatic in the presence of a diene rubber having at least one stable free radical group under polymerization conditions such that a vinyl aromatic-diene block and/or graft copolymer rubber is formed. This patent does not disclose the use of acrylic initiators.
U.S. Pat. No. 5,627,248 discloses a living free radical polymerization process for vinyl aromatic monomers, which employs a difunctional nitroxyl initiator. The dinitroxide compounds described in the patent comprise TEMPO or TEMPO-based derivatives. This class of free radical control agent does not provide control over acrylic type monomers. Specifically, the use of methacrylics will lead to side and termination reactions such as disproportionation, which inhibits the formation of block copolymers and long chain molecules (as described by Ananchenko et. al. in the Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 40 pp 3264-3283). Therefore dinitroxyl acrylic based macroinitiators cannot be produced directly from dinitroxides such as TEMPO and TEMPO-based alkoxyamines. Such are not suited to the controlled polymerization of acrylics. The other methods described in U.S. Pat. No. 5,627,248 for forming telechelic dinitroxyl macroinitiators are plagued by inefficiency, i.e., while some fraction of telechelic materials will form, another fraction will contain only mono-nitroxide functionality and in some cases a fraction of the polymer will have no nitroxide functionality. For example, the formation of telechelic macroinitiators starting from dinitroxyl azo, sulfide or peroxide compounds are described. This method relies on termination via chain coupling to produce a telechelic macroinitiator. According to Odian, G.; in Principles of Polymerization, Fourth Edition, John Wiley & Sons, Inc., 2004, approximately 10% of peroxide termination occurs via disproportionation and chain transfer. This would lead to a yield of less than 85% of the described dinitroxide comprising TEMPO or TEMPO-based derivatives. The macroinitiators of the present invention yield much greater than 85% dinitroxide owing to the fact that peroxide moieties are not required.
U.S. Pat. No. 6,255,402 discloses a process for preparing a vinyl aromatic polymer matrix and particles of rubber by polymerizing a vinyl aromatic in the presence of a rubber comprising a group which generates a stable free radical. The introduction of a poly(meth)acrylate macroinitiator into a standard polystyrene process is not disclosed in the above patent.